Production of hollowed/channeled protective thermal-barrier coatings functioning as heat-exchangers

ABSTRACT

A process for production of hollow thermal-barrier coatings functioning as high-temperature heat-exchangers made of metal and/or non-metallic materials, applied onto the surface of metal and/or non-metallic substrata. The hollow/channeled coatings having a predetermined design, comprising an outer shell and an inner framework, which framework joins the outer shell with the surface of the substrate, together making a system of hollows and/or channels in the space between the outer shell and the substrate. The process includes: 1) preparing the substrate surface for application of materials; 2) forming auxiliary extractable elements to model required hollows and channels in the coating; 3) applying a layer of material, constructing the inner framework and/or an outer shell, by one of the following methods: physical or chemical vapor deposition, sputtering in a vacuum and thermal spray processes, or by chemical or electrochemical deposition, or by any combination thereof, 4) repeating the cycle of the preceding steps to produce all the tiers of the coating; 5) final extraction out of the coating of the auxiliary extractable elements by any existing methods, such as by sublimation in a vacuum. The present process allows the production of hollow coatings with at least two independent systems of hollows and channels. One system is designed for heat-carrier passage, and another for protective inert gas, to reduce oxidation in high-temperature components. In a particular embodiment, a two-tiered hollow/channeled coating containing channels for a heat-carrier is formed by application of a material through the process of physical vapor deposition.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention pertains to the field of coatings for high-temperaturestructural materials. This invention comprises the technology, wheremainly physical vapor deposition, sputtering in a vacuum and thermalspray processes, are used for production of hollow protectivethermal-barrier coatings functioning as high-temperatureheat-exchangers, which coatings can be used, e.g., for thermal barrierprotection of gas turbine engine blades and other components againstdegradation by high-temperature oxidation and hot corrosion.

2. Background

Generally, the following described process is used to fabricate asurface heat-exchanger on the substrate of a component: open channelsfor heat-carrier are made on the surface of the component's material(substrate), then these channels are filled by an extractable filler tothe surface level, and the whole of the obtained surface is covered byan unbroken/continuous layer of a material, making an outer shell (thecover), covering the channels, after which the filler is extracted fromthe channels, and a surface heat-exchanger, as described below, isobtained, consisting of an outer shell, covering open channels cut intothe substrate body. The shaping of the open channels in the substratebody is produced by various techniques, e.g., during the casting of thecomponent, or formed by mechanical or electromechanical machining. Thechannels are filled by an extractable filler, usually, in a paste form.The filler prevents the deposited material of the outer shell fromgetting into the channels. The formation of the outer shell may beaccomplished using various techniques of coating application, e.g.,thermal spray, or physical vapor deposition. The material, of which theouter shell is formed, is chosen based on the specific functions to becarried out by the protected component, (e.g., UK Patent Application GB2172060 A, INT CL F01D 5/00, by Rolls Royce Ltd.; Germany, PatentApplication DE 37 06 260, INT CL F 01 D 5/18, by Siemens AG; Japan,Patent Application 61-25 881, INT CL F 01 D 5/18).

In the prior art, surface heat-exchangers are used in various fieldswhen a decrease or an increase of surface temperature of a component isrequired; chiefly of specific components operating in severe temperatureand/or mechanical stress conditions. Also known and used is the methodof mounting ready-made heat-exchangers onto the surface of a component.

Usually the elements of such heat-exchangers are produced by thestamping technique and are fixed onto a component surface by solderingor welding. But such heat-exchange surfaces are complex to manufactureand unreliable in operation, which is why they are limited for practicaluse and cannot be used for gas-turbine engine (GTE) blades, and similarcomponents operating in severe temperature and mechanical stressconditions. Development of effective, reliable surface heat-exchangerspossessing a range of protective coating properties is a continualchallenge in airspace, power-generation, and rocket technologies.

To adequately describe and illustrate the problem being considered,observation of the use of such surface heat-exchangers for protection ofGTE blades from overheating will suffice.

In modern GTE and GTU (gas-turbine units), operational temperatures aremore than 1000° C. Improving economic operation and efficiency of agas-turbine requires an increase in operating temperatures, which couldbe provided either by cooling GT components during operation, or byusing materials with higher temperature and stress resistant properties.

PRIOR PROTOTYPE EXAMPLE

One prior prototype of a surface heat-exchanger is described as a shell(sheath) covering open channels, which are cut into the body of thesubstrate (the body of the protected component). This type of device andthe required technology have a number of significant drawbacks, whichlower the quality and operational properties of a surface heat-exchange.

Discussion of drawbacks.

1. In this prototype the partitions between the channels, which are,actually, the partitions of a heat-exchanger, must be wide enough toprovide efficient cohesion of the shell with the protected component(the area of the partitions' end-face).

If the areas of cohesion are increased to provide better adhesion of theshell, then areas for cooling channels are decreased, leading to thefollowing disadvantageous consequences:

 the channel flow area decreases;

 severe and debilitating temperature fluctuations occur within the shellat the conjunction points of cooling channels and adhesion surfaces;

 a high level of heat transfer from the shell to the substrate via thepartitions, which decreases the protective properties of the shell.

2. The presence of channels in the body of the substrate (in the body ofthe blade itself) probably greatly reduces its mechanical strength,acting as a concentrator of mechanical tensions.

3. The only type of surface heat-exchanger possible to produce by thistechnology is the one described above as a prototype.

4. A multi-tier heat-exchanger is impossible to produce using suchtechnology.

5. It is extremely difficult to vary width and configuration of thechannels and partitions cross-sections.

6. It is impossible to apply a ceramic coating onto the shell, due tothe severe temperature fluctuations within the shell.

SUMMARY OF THE INVENTION

This invention deals with basic principles of the production of hollowthermal-barrier coatings functioning as heat-exchangers, which may beapplied for various design embodiments. The coatings consist of an outershell and an inner framework, joining the outer shell with the surfaceof the substrate and building up a system of hollows and/or channels inthe space between the outer shell and the substrate. In other words, thecoatings which can act as heat-exchangers are produced by certainmethods of the application of materials, as described below:

1) preparation of the surface of the substrate for application ofmaterials;

2) modeling the hollows and/or channels by application of the auxiliaryextractable elements onto the surface of the substrate, which auxiliaryelements are made of extractable filler(s);

3) application of the layer of the material of the coating, to form theinner framework on the obtained surface (the obtained surface beingcomprised of open areas of the substrate (and/or bondcoat) and that ofauxiliary elements. Said obtained surface achieved as the result ofrepeated previous cycles (p.1, 2) will consist of the applied materialof the coating and auxiliary elements, and/or a combination thereof).

4) repetition of steps 2 and 3 to complete formation of the framework(preceded by appropriate preparation of the obtained surface);

5) application of the layer of the main material of the coating, formingthe outer shell, onto the appropriately prepared surface, which surfaceis a common surface comprised of the surfaces of the open areas of theframework and the surfaces of the auxiliary elements;

6) final extraction of the auxiliary elements from the assembledbody/central portion of the coating.

The object of the present invention is to improve the quality andfunctional properties of surface heat-exchangers with protective outershells, due to the elimination of the aforementioned drawbacks by meansof:

1. Transpositioning the channels/hollows for a heat-carrier directlyinto the body of the protective coating.

2. Significant decrease of the partition thickness between thechannels/hollows, which will allow

a nearly doubled capacity of the flow area of the channels retainingdesired height;

equalization of the temperature within the shell and on the surface ofthe component;

decrease of heat transfer from the shell to the component, and anincrease of heat transfer from the shell to the heat-carrier.

3. Increase in area and strength of surface cohesion.

4. The strength of the protected component is improved because thetechnology of this invention does not require channels being cut intothe body of the component.

5. Ability to form both single and multi-tier coatings functioning asheat-exchangers.

These and other objects and advantages of this invention are describedin detail in the description of the invention contained herein.

BRIEF DESCRIPTION OF THE DRAWING

The invention may better be understood by reference to the followingdetailed description of a preferred embodiment, which is given hereafterin conjunction with several figures of the drawing in which:

FIG. 1 is a cross sectional view of the layers during production of ahollow/channeled coating in accordance with the present invention.

FIG. 2 is a perspective view (partially cut away) of a coating formed inaccordance with the present invention.

FIG. 3 is a cross sectional/view of the layers during production of ahollow/channeled coating with bond coat.

FIGS. 4(a, b, c, d, e) is a perspective view (partially cut away) ofcoatings obtained after repeated cycles of production.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

As the basis of the preferred design of the proposed hollow/channeledcoating functioning as a heat-exchanger, the traditional lamellar typeheat exchanger is taken.

Referring to FIG. 1, the principal steps in the production of ahollow/channeled coating that functions as a heat exchanger are shown.The consequence of the coating formation steps is as follows:

1. The substrate 1 (the surface of the component to be protected) isprepared for materials application in accordance with the chosen methodof coating application;

2. The auxiliary elements 2 of the first tier formation are formed of anextractable filler on the surface of the substrate 1, as shown in FIG.1A;

3. The coating main material is applied onto the obtained surface as acontinuous layer to obtain the inner framework 3 (or partitions), asshown in FIG. 1B;

4. The cycle of steps 2 and 3 is repeated to form a second tier of thecoating, in which the second tier auxiliary elements 2 are formed on theinner framework surface 3 (FIG. 1C) and the coating main material layeris applied, to form an outer shell 4 of the coating in this particularcase (FIG. 1D); and

5. The auxiliary elements 2 are extracted, and a complete coating isobtained with inner hollows or channels 5 (FIG. 1E).

FIG. 3 shows the same steps as in FIG. 1 with bond coat 6.

The preparation of the surface of the substrate 1 for the materialapplication may be accomplished by various methods, the selection ofwhich is based on the choice of the material application method and theproperties of the material being applied. Preparation may includeapplication of a bondcoat 6, which in some cases may act as solder forbetter adhesion of the materials both of the substrate and the coating.

Application of the extractable auxiliary elements on the surface of thesubstrate 1 may be accomplished by one of the following methods:

moldering (molding, sintering, stamping) the auxiliary elements onto thesubstrate surface by means of a die, which have moldering cells,providing the required shape, dimensions, distribution and location ofthe auxiliary elements 2;

application of the layer of the material of the extractable filler ontothe surface through a mask attached to the surface, which mask hasopenings (holes/cuts) located and shaped so as to model location,distribution and shape of the auxiliary elements, which application ofthe layer of the material of the extractable filler can be accomplished,e.g. by thermal spraying of a powder-like material applied with thethickness equal to the required height of the auxiliary element 2.

application of the filler in a layer to the required thickness (equal tothe predetermined height of the auxiliary element 2), followed by theremoval of unneeded portions of the filler by one or more of thefollowing techniques: either by local heating and removal due tosublimation of the unneeded portions of the filler, e.g. by ahigh-energy beam (a laser beam) or by application of a heated molder,providing the local removal of the material in the heated zones; or byone of the known methods of physical, chemical, electrochemical orplasma etching, or mechanical machining.

The layer of the material of the extractable filler is applied to athickness in the range between 0.0001-10,000 microns. This provideshollows in the finished structure with dimensions in the range between0.0001-10,000 microns.

Application of the main material(s) layer, forming the inner framework 3and the outer shell 4, to the obtained surface, may be done by one ofthe following methods or by their combination: physical or chemicalvapor deposition, sputtering in a vacuum and thermal spray processes,chemical or electrochemical deposition. The choice of method ofdeposition of the coating main material is based on certain and/orparticular properties of the coating, on suitability of a specifiedmethod for a particular protected component, and, also, on economicconsiderations. These conditions are also applicable for the depositedmaterial choice. The coating material forming the inner framework and/orthe outer shell may be chosen from a variety of high-temperaturecoatings, thermal-barrier coatings, corrosion-resistant coatings,erosion-resistant coatings, or wear-resistant coatings. For example, thecoating material may be a high-temprature coating such as MCrAlY.Alternatively, the coating material may be a thermal-barrier materialsuch as ceramic. Choices of protective coating technologies are wellknown; thus, it is not necessary to consider them in further detail. Theouter shell 4 is formed to a thickness in the range of 0.01-1000microns. The thickness of the coating layer forming the inner framework3 is in the range of 0.001-1000 microns.

So as to provide a higher quality of the coating, especially when thephysical vapor deposition is used, prior to application of the auxiliaryelements onto the substrate surface, and also prior to the applicationof the main material of the coating, a metal (alloy) bondcoat of 1-60microns thickness is applied, which in some cases may act as solder forbetter adhesion of the materials both of the substrate and the coating.

With the described coating formation process, extractable filler(s) isto be used. After completing the formation of all the layers of thecoating, the filler is extracted from the coating by one of thefollowing techniques, depending on the properties of the chosen filler:by its sublimation in vacuum when heated (combined with thermaltreatment of the coating); by one of the known dissolving methods:physical, chemical, or electrochemical, etc. The auxiliary elements aremade of a material capable to sublimate in a vacuum at a temperaturelower than the temperature that would cause degradation of operationalfeatures of the coating that forms the inner framework 3 and/or outershell 4.

Metal fluoride compounds may be used for the materials choice of theextractable filler: ZrF₄, BeF₂, NaF, CrF₃, CoF₂, MgF₂, BaF₂, CaF₃, FeF₂,or metal chlorides: KCl, or metal oxides: BaO.

According to the preferred embodiment of the invention (in fact, therecould be many embodiments), a two-tiered hollow/channeled coating isformed on a protected component (i.e., having two tiers of channels).The auxiliary elements are formed by stamping. The main material of thecoating is applied by physical vapor deposition; the bondcoat, by gasthermal method.

As the basis of the preferred design of the proposed protectivethermal-barrier coating functioning as heat-exchangers, the traditionallamellar heat-exchanger is used as a prototype. The present processallows the production of hollow coatings with at least two independentsystems of hollows and channels (FIG. 2, FIG. 4). One system is designedfor heat-carrier passage, and another for protective inert gas, toreduce oxidation in high-temperature components. As illustrated, theprocess produces an inner framework 3 that has a shape of a zigzagpartition of trapezoid cross-section.

An additional coating, such as a layer of ceramic thermal-barriercoating, may be applied on top of the structure shown. The additionalcoating may have a thickness in the range between 0.0111-20,000 microns.

The described consequence of these steps was realized to obtain acoating functioning as a heat-exchanger on a gas turbine blade made of ahigh-temperature Ni based super alloy.

There were already existing outlets for operational cooling air supplyin the substrate. The surface preparation for coating was done by airsand blower machining with micro dispersed corundum. As an auxiliaryelements material sodium fluoride (NaF) was used, having the sublimationtemperature T=693° C. under the vacuum 0.0001 mm Hg. The filler wasapplied onto the surface in correspondence with the required forms ofthe channels to be obtained. As the coating main material ahigh-temperature resistant alloy of NiCrAlY system was used, appliedaccording to the traditional technology of application of protectivecoatings onto GTE blades. Prior to the application of the auxiliaryelements onto the substrate surface, and, also, prior to the maincoating material application, a bondcoat of high-temperature resistantsolder was applied. The bondcoat was applied by plasma spraying. Thebondcoat thickness was 20 microns. The inner framework thickness was 180microns, the outer shell—200 microns. The height of the auxiliaryelements was 500 microns. The filler extraction was done by its vacuumsublimation combined with thermal diffusion annealing of the coating ina vacuum furnace under the temperature of 1080° C. for four hours.

FIG. 2 presents a view of the coating, obtained in accordance with theconsidered particular example. The design of the obtained version is asfollows: the outer shell 4, lying on the corrugated framework 3,dividing hollow/channeled space between the substrate 1 and the shell 4into the system of channels (hollows) 5.

While this invention has been described in terms of a specificembodiment thereof, it is to be understood that it is not limitedthereto, but rather only to the extent set forth hereafter in the claimswhich follow.

I claim:
 1. A process for production of a hollow coating applied onto asubstrate surface, said coating having a pre-determined design ofhollows, which process includes preparation of said substrate surfacefor application of said coating and at least one cycle consisting of thefollowing steps (a) and (b): (a) modeling said hollows by formation ofauxiliary elements on said substrate surface, or, if said cycle isrepeated, modeling said hollows by formation of auxiliary elements ontothe surface of a previous layer of said coating; said auxiliary elementsmodeling dimensions, shapes, and location of said hollows relative tosaid substrate; (b) application of a layer of pre-determined thicknessof desired material of said coating onto an obtained surface; saidobtained surface comprising said auxiliary elements in combination withopen areas of said substrate not covered by said auxiliary elements, or,if said cycle is repeated, said obtained surface comprising saidauxiliary elements in combination with open areas of said previous layerof said coating not covered by said auxiliary elements; and extractionof said auxiliary elements after a pre-determined number of repetitionsof said cycle.
 2. A process for production of said coating as claimed inclaim 1, wherein said formation of said auxiliary elements is done by aprocess for production of extractable models.
 3. A process forproduction of said coating as claimed in claim 2, where said process forproduction of extractable models comprises applying materials of saidauxiliary elements onto said substrate surface or onto said previouslayer of said coating.
 4. A process for production of said coating asclaimed in claim 3, where said step of applying material is physicalvapor deposition and/or chemical vapor deposition and/or sputteringand/or thermal spray processes.
 5. A process for production of saidcoating as claimed in claim 4, where said auxiliary elements areproduced by the following steps: a) by application of a solid layer ofmaterial of said auxiliary elements onto said substrate or said previouslayer with a thickness of said solid layer equal to a pre-determinedheight of said auxiliary elements, and b) by application of apre-determined shape to said auxiliary elements by means for the removalof unneeded material of said auxiliary elements.
 6. A process forproduction of said coating as claimed in claim 3, where said step ofapplying material is done through a mask, said mask modeling dimensionsand location of said auxiliary elements.
 7. A process for production ofsaid coating as claimed in claim 6, where said auxiliary elements aremade of material capable to sublimate at a temperature lower than thetemperature that would cause degradation of operational features of saidcoating.
 8. A process for production of said coating as claimed in claim7, where said auxiliary elements are made of material, capable tosublimate in a vacuum at a temperature lower than the temperaturecausing degradation of operational features of said coating.
 9. Aprocess for production of said coating as claimed in claim 8, where saidmaterial, of which said auxiliary elements are made, is made of at leastone of the following compounds: ZrF₄, BeF₂, NaF, CrF₃, CoF₂, MgF₂, BaF₂,CaF₃, FeF₆, KCl, BaO.
 10. A process for production of said coating asclaimed in claim 1, where said application of said material of saidcoating is done by means for application of coatings.
 11. A process forproduction of said coating as claimed in claim 10, where said means ofsaid application of said material of said coating is one of thefollowing methods and/or any combination thereof: physical or chemicalvapor deposition, sputtering in a vacuum, thermal spray processes, orchemical or electrochemical deposition.
 12. A process for production ofsaid coating as claimed in claim 10, where said means of saidapplication of said material of said coating is a method for applicationof high-temperature coatings, thermal-barrier coatings,corrosion-resistant coatings, erosion-resistant coatings, orwear-resistant coatings.
 13. A process for production of said coating asclaimed in claim 1, where said coating consists of an outer shell and aninner framework, which said inner framework joins said substrate surfacewith said outer shell and divides a space between said outer shell andsaid substrate surface in a system of said hollows.
 14. A process forproduction of said coating as claimed in claim 13, where saidpre-determined design of said hollows is a design of a heat-exchanger.15. A process for production of said coating as claimed in claim 14,where said preparation of said substrate surface includes application ofauxiliary bond coat.
 16. A process for production of said coating asclaimed in claim 15, where, after completion of said coatingapplication, a thermal-barrier coating is applied onto said outer shell.17. A process for production of said coating as claimed in claim 15,where said hollows constitute at least two independent systems, one ofsaid systems being designed for a heat-carrier, the other being designedfor inert, protective gas.
 18. A process for production of said coatingas claimed in claim 13, where said inner framework has a shape of azigzag partition of trapezoid cross-section.
 19. A process forproduction of said coating as claimed in claim 13 where said preparationof said substrate surface includes application of auxiliary bond coat.20. A process for production of said coating as claimed in claim 19,where, after completion of said coating application, a thermal-barriercoating is applied onto said outer shell.
 21. A process for productionof said coating as claimed in claim 13, where, after completion of saidcoating application, a thermal-barrier coating is applied onto saidouter shell.
 22. A process for production of said coating as claimed inclaim 13, where said outer shell thickness range between 0.01-1000microns, said framework thickness range between 0.001-1000 microns,cross-section dimensions of said hollows range between 0.0001-10000microns, and said coating thickness range between 0.0111-20000 microns.23. A process for production of said coating as claimed in claim 13,where said hollows constitute at least two independent systems, one ofsaid systems being designed for a heat-carrier, the other being designedfor inert, protective gas.
 24. A process for production of said coatingas claimed in claim 1, where said preparation of said substrate surfaceincludes application of auxiliary bond coat.
 25. A process forproduction of said coating as claimed in claim 24, where said coating isapplied to an airfoil of a gas-turbine.